The present invention relates to a rotary displacement compressor having at least one rotor mounted in a working space formed by a casing surrounding the rotor(s), in which working space a gaseous working fluid is transported and compressed in compression chambers from an inlet port connected to a suction channel to an outlet port connected to a pressure channel, said outlet port having an edge determining the moment when a compression chamber opens towards the outlet port, the position of said edge being adjustable by actuating means governed by sensing means.
The internal compression of a compressor is independent of the pressure in the pressure channel and is for a certain working fluid depending only on the volume ratio of the compressor, i.e. the relation between the volume of a compression chamber at the moment it just has been closed off from the inlet port and the volume of a compression chamber at the moment just before it is opened towards the outlet port. Assuming a constant inlet pressure, a certain volume ratio thus results in a certain pressure in a compression chamber just before it is opened towards the outlet port, the end pressure of the compressor. It is desirable that the internal compression corresponds to the pressure in the pressure channel, so that the pressure in a compression chamber just before it opens towards the outlet equals the pressure in the pressure channel. If these pressures differ from each other, i.e. at over- or undercompression, a rapid flow of gas through the outlet port occurs each time a compression chamber opens towards it, whereby the pressures become equalized. The flow velocity during this short moment is much higher than the flow velocity of the working fluid when it is displaced out through the outlet port by the rotors, and the direction thereof can be to or from the pressure channel depending on if there is over- or undercompression. These flow pulses generate disturbing noise and vibrations, which can damage the connected pipe system. Simultaneously the efficiency of the compressor will decrease. By these reasons there is always an effort to adapt the built-in volume ratio to the pressure in the pressure channel.
In some cases this pressure, however, can vary, which under such condition makes it desirable to correspondingly make it possible to vary the volume ratio. Since long it is therefore known to provide a compressor with devices regulating this, so called V.sub.i -regulation. This is accomplished in that the position of the edge of the outlet port, which determines the moment of opening, can be varied in steps or continuously. By this the volume of a compression chamber at the moment of opening can be changed and therewith the volume ratio. In this way it can be achieved that the pressure in said compression chamber roughly equals the pressure in the pressure channel.
Constructively this can be made in many ways, partly depending on which kind of rotary displacement compressor it relates to. On e.g. a rotary screw compressor having two cooperating rotors, a frequently used regulating device consists of an axially movable slide, displaceably mounted in guiding means parallel to the rotors. The slide has a surface facing the working space, which surface forms a part of the barrel wall of the working space and complies with its shape. The end of the slide facing the high pressure end of the compressor is provided with an edge forming an edge of the outlet port. When the position of said edge is changed by displacement of the slide, the moment of opening of a compression chamber towards the outlet port will be changed and with this its volume at that moment.
For adjusting the slide to a correct position, where neither under- nor overcompression prevails, it is known to have the slide position influenced by sensed operating parameters of the compressor. Examples of such devices are disclosed in SE No. 427 063, SE No. 430 709, DD No. 127 878 and U.S. Pat. No. 3,936,239. The operating parameters sensed in the compressors disclosed in the above mentioned patent documents are either the electrical power consumption of the prime mover or the difference between the outlet pressure and the pressure in a compression chamber just before opening. In the first alternative the slide is adjusted to a position where the power consumption is at its minimum, which corresponds to a minimum of losses in efficiency due to under- or overcompression. In the second alternative the pressure in the compression chamber affects the slide to move in a direction of larger outlet area, whereas the pressure in the pressure channel affects the slide to move in the opposite direction, whereby the slide is adjusted to a position where these pressures balance each other.
Both these methods for governing the V.sub.i -regulation, however, have serious deficiencies.
Using the power consumption as the governing parameter introduces a source of error in that fluctuations in the electricity supply network affects the sensed parameter. Furthermore, the power consumption as a function of the deviations of the end pressure in the compressor from the pressure in the pressure channel has a very flat characteristic, resulting in a poor accuracy; which allows the influence of said fluctuations to be relatively dominating. This method of governing therefore at the best might be able to keep the losses in efficiency at an acceptable level but will not be sufficient to handle the noise problem.
The method to use the pressure difference for governing the regulation has shown to be difficult to work in practice. The main reason for that is that sensing the end pressure in the compressor cannot be accomplished in a reliable way, since the sensed pressure fluctuates, and considerable pressure pulses are generated each time the means limiting a compression chamber passes the sensing point. It will therefore be practically impossible to use this way for reaching the balanced position where neither under- nor overcompression prevails.